This invention relates to a lithium ion secondary battery employing a thin film solid electrolyte and a method for manufacturing the same.
In the past, a non-aqueous electrolytic solution was generally used as an electrolytic solution for a lithium ion secondary battery. A lithium ion secondary battery employing a polymer electrolyte made of polymer as disclosed by Japanese Patent Application Laid-open Publication No. 2000-067917 has recently attracted more attention of the industry than such electrolytic solution employing liquid.
The lithium ion secondary battery employing a polymer electrolyte holds a liquid electrolytic solution in the polymer electrolyte and, therefore, has the advantage that there is little possibility of leakage of the liquid, that there is little possibility of corrosion, that short-circuiting between electrodes caused by precipitation of lithium in the form of dendrite can be prevented and that assembly of the battery is easy because the structure of the battery is very simple.
Since lithium ion conductivity of such polymer electrolyte is lower than an electrolyte containing only an electrolytic solution, there has occurred a practice to reduce thickness of the polymer electrolyte. There, however, has arisen a problem in such polymer electrolyte whose thickness is reduced that, since its mechanical strength is reduced, the polymer electrolyte tends to be broken or give rise to a hole during production of the battery resulting in short-circuiting between the positive electrode and the negative electrode. The gel polymer electrolyte is reported to have thickness in the order of 30 μm to 80 μm.
For improving the mechanical strength, there is a proposal in Japanese Patent Application Laid-open Publication No. 2001-015164 for a compound electrolyte containing lithium ion conductive glass-ceramic powder. This proposal however has not realized a thin film electrolyte having thickness of 20 μm or below.
There are also many proposals, e.g., in Japanese Patent Application Laid-open Publication No. Hei 07-326373, for a solid electrolyte battery which does not employ an electrolytic solution at all. Since a lithium ion secondary battery employing a solid electrolyte does not require an organic electrolytic solution as in the prior art batteries, there is no risk of leakage of solution and combustion and, therefore, a highly safe battery can be provided. In the prior art battery employing an organic electrolytic solution, the positive electrode and the negative electrode contact each other by means of the organic electrolytic solution through the solid electrolyte and, therefore, resistance in moving of ions in the interface does not cause a serious problem. If, however, all of the positive electrode, negative electrode and electrolyte composing the battery are made of solid, contact in the interface between the positive electrode and the electrolyte and contact in the interface between the negative electrode and the electrolyte become contacts between solids which include point contacts in some parts of the interfaces and thereby produce a large interface resistance as compared with the prior art batteries employing the electrolytic solution. Hence, the solid electrolyte battery has a large impedance in the interfaces which tends to cause polarization and thereby restrict moving of lithium ion in the interfaces with the result that it is difficult to realize a battery having a large capacity and a large output by such solid electrolyte battery.
It is, therefore, an object of the present invention to provide a lithium ion secondary battery which has solved the above problems and has a thin electrolyte and thereby has small resistance notwithstanding that a solid electrolyte is employed and, therefore, has a high battery capacity and a high output and an excellent charging-discharging characteristic and thereby ensures a stabilized use over a long period of time.